A wireline with multiple internal conductors can be used to transport a logging tool downhole and to transmit data between the tool and surface equipment. The typical wireline has a single insulated inner conductor wrapped by a helix of six insulated conductors. Layers of armor strands wrapped in opposing directions overlay these conductors.
In most instances, each conductor is assigned to a single dedicated analog or digital signal. Unfortunately, the wireline tends to attenuate transmitted signals (consisting of a voltage difference between two terminals) from the wireline's input to its output. The attenuation typically increases with frequency and results from the resistance, capacitance, and inductance of each individual conductor carrying the signal. Also, crosstalk can occur in the conductors through mutual impedance, when a signal transmitted on one set of conductive paths generates a corresponding signal on other paths. Again, the presence of crosstalk also increases with frequency.
Prior art configurations have been developed to make multi-conductor wirelines more useful in carrying a greater number of data signals and to increase the maximum frequency bandwidth of these signals. Such prior art configurations have been designed for Triaxial Borehole Seismic (TABS) downhole logging system used in microseismic hydraulic fracture mapping. An example of such as system is disclosed in U.S. Pat. No. 5,747,750 to Bailey et al., which is incorporated herein by reference in its entirety. With the TABS system deployed downhole, operators inject frac fluids at fracturing rates into the formation, and the TABS system records microseismic acoustic data emitted by the formation in response to the hydraulic fracture stimulations. The TABS tool then transmits the microseismicity through the wireline to the surface where it can then be interpreted to provide a three-dimensional mapping of the fracture network.
Yet, problems with attenuation and cross-talk, especially at higher frequencies, can still limit the available bandwidth for the signals transmitted uphole with the above system. Therefore, a downhole telemetry configuration disclosed in U.S. Pat. No. 7,026,951 to Bailey et al., which is incorporated herein by reference in its entirety, has been developed to handle such problems. The configuration reproduced in FIG. 1 includes a wireline 10 having seven conductors (1-7). One end of the wireline 10 couples to a transformer 20 for the telemetry input of a downhole TABS tool (not shown), while another end of the wireline 10 couples to a transformer 30 for the telemetry output to a data acquisition system (not shown). One terminal 22a/32a of each transformer 20/30 connects to cross-wise or opposing pairs of conductors (1 & 4), while the other terminals 22b/32b connect to cross-wise pair of conductors (2 & 5). Connecting the conductors in these cross-wise pairs improves the frequency response of the telemetry configuration.
The configuration in FIG. 1 is operated using a frequency modulated telemetry scheme. In this scheme, geophones of the TABS tool generate data signals, and the downhole telemetry unit digitizes the data signals and modulates the digitized signals into frequency modulated (FM) signals. Then, the downhole telemetry unit transmits the FM signals uphole via the multi-conductor wireline 10. At the surface, the data acquisition system coupled to the wireline 10 receives the FM signals and demodulates them so they can be processed to determine the microseismic acoustic data.
Although the configuration of FIG. 1 operated using a frequency-modulated telemetry scheme is effective, operators are continually seeking increased functionality when performing logging operations.